Silicide compositions containing alkali metals and methods of making the same

ABSTRACT

The invention relates to a method of making alkali metal silicide compositions, and the compositions resulting from the method, comprising mixing an alkali metal with silicon and heating the resulting mixture to a temperature below about 475° C. The resulting compositions do not react with dry O 2 . Also, the invention relates to sodium silicide compositions having a powder X-ray diffraction pattern comprising at least three peaks with 2Theta angles selected from about 18.2, 28.5, 29.5, 33.7, 41.2, 47.4, and 56.2 and a solid state  23 Na MAS NMR spectra peak at about 18 ppm. Moreover, the invention relates to methods of removing a volatile or flammable substance in a controlled manner. Furthermore, the alkali metal silicide compositions of the invention react with water to produce hydrogen gas.

RELATED APPLICATION DATA

This application claims benefit of priority of Provisional Application.Ser. Nos. 60/578,818 filed Jun. 14, 2004 and 60/667,693 filed Apr. 4,2005, the entire disclosures of which are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to alkali metal silicide compositions made by theinteraction of alkali metals with silicon at temperatures below about475° C. The compositions provide a stable source to reduce water,producing a source of pure hydrogen gas.

BACKGROUND OF THE INVENTION

Alkali metals are very reactive in their metallic or neutral state.Alkali metals are very reactive toward air and moisture and may catchfire spontaneously when exposed to these agents. To avoid the inherenthazards associated with their activity, the neutral metal must often bestored in vacuo or under an inert liquid such as oil in order to protectit from contact with the atmosphere, which may result in oxidation orother reactions. For example, sodium metal is often stored in Nujol oilwhich must, to avoid unwanted impurities, be removed prior to use inchemical reactions. This places severe restrictions on its shipment anduse.

A number of compounds between alkali metals and silicon compounds havebeen prepared. For example, known compounds between sodium (Na) andsilicon (Si) exist with stoichiometries that range from NaSi to Na(Si)₆(which is believed to be Na₈Si₄₆) to Na_(x)Si₁₃₆, with 1.5<x<24. (SeeWitte, J.; Schnering, H. G., “The Crystal Structure of NaSi and NaGe (inGerman)” Zeit Anorgan Allege Chemie 1964, 327, 260-273., Cros, C.;Pouchard, M.; Hagenmueller, P., “Two new Phases of the Silicon-SodiumSystem. (in French)” C. R. Acad. Sc. Paris 1965, 260, 4764-4767., andHe, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse,J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J.Phys. Chem. B 2001, 105.). The known compounds are formed by heating Nawith Si to high temperatures, always at or above 500° C., and in somecases with removal of Na vapor by condensation on a cold surface. (SeeHe, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse,J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J.Phys. Chem. B 2001, 105. and Mayeri, D.; Phillips, B. L.; Augustine, M.P.; Kauzlarich, S. M., “NMR Study of the Synthesis of Alkyl-TerminatedSilicon Nanoparticles from the Reaction of SiCl₄ with the Zintl Salt,NaSi” Chem. Mater. 2001, 13, 765-770.). Mayeri et al. react silicon inthe presence of sodium to a temperature of about 650° C. to form asodium silicide. There have also been reports that a silicide of nominalcomposition NaSi₂ can be prepared by heating Na with quartz (SiO₂),although the evidence for this composition is slim. (See Novotny, H.;Scheil, E., “A Ternary Compound in the System Aluminum-Silicon-Sodium(in German)” Metallforsch. 1947, 2, 76-80.).

It has often been assumed that NaSi is so reactive that it must beconsidered to be pyrophoric, or able to spontaneously ignite in thepresence of air. It has also been recently characterized as “air andmoisture sensitive.” (See He, J.; Klug, D. D.; Uehara, K.; Preston, K.F.; Ratcliffe, C. I.; Tse, J. S., “NMR and X-ray Spectroscopy ofSodium-Silicon Clathrates” J. Phys. Chem. B 2001, 105.). This studyshowed, however, that the clathrate structure of Na₈Si₄₆ is non-reactivetoward air and moisture. Id. However, it is generally agreed that thereaction of NaSi with water is rapid and “violent”, such that the heatof reaction can ignite the hydrogen formed, just as occurs in thereaction of alkali metals with water. This places severe restrictions onstoring and handling NaSi without keeping it in vacuo or under an inertatmosphere to avoid its inherent hazards.

A major problem with the synthesis of NaSi materials has been the needto heat Na and Si in a closed system to prevent the condensation of Naat cold sites. Conventionally, for example, to prepare polycrystallineNaSi powder, excess Na was heated with Si in a molybdenum (Mo) tube thatwas welded shut and heated for three days at 500° C. (See Mayeri, D.;Phillips, B. L.; Augustine, M. P.; Kauzlarich, S. M., “NMR Study of theSynthesis of Alkyl-Terminated Silicon Nanoparticles from the Reaction ofSiCl₄ with the Zintl. Salt, NaSi” Chem. Mater. 2001, 13, 765-770.). Inanother study, a stainless steel container was used. (See He, J.; Klug,D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., “NMRand X-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem. B2001, 105.).

A need exists, therefore, to prepare an alkali metal silicidecomposition conveniently and inexpensively, so that it may be easilyhandled in air without a significant loss in its ability to reducewater. This reduction reaction would be able to produce large amounts ofhydrogen per unit mass of the solid.

SUMMARY OF THE INVENTION

The invention relates generally to alkali metal silicide compositions,methods of making the alkali metal silicide compositions, and methods ofusing the alkali metal silicide compositions. Any alkali metal may beused in the practice of this invention, including Sodium (Na), Potassium(K), Rubidium (Rb), and Cesium (Cs).

Specifically, the invention relates to an alkali metal silicidecomposition comprising the product of mixing an alkali metal withpowdered silicon in an inert atmosphere and heating the resultingmixture to a temperature below about 475° C., wherein the alkali metalsilicide composition does not react with dry O₂. In this embodiment, thealkali metal silicide composition may be sodium silicide, such asNa₄Si₄, potassium silicide, such as K₄Si₄, and the like.

In addition, the invention relates to a sodium silicide compositionhaving a powder X-ray diffraction pattern comprising at least threepeaks with 2Theta angles selected from about 18.2, 28.5, 29.5, 33.7,41.2, 47.4, and 56.2. Furthermore, the invention relates to a sodiumsilicide composition having a solid state ²³Na Magic Angle Spinning(MAS) Nuclear Magnetic Resonance (NMR) spectra peak at about 18 ppm. Inthese embodiments, the sodium silicide may be Na₄Si₄, for example.

Moreover, the invention relates to a method of removing a volatile orflammable substance in a controlled manner, the volatile or flammablesubstance being in the presence of water, the method comprising the stepof exposing the volatile or flammable substance to an alkali metalsilicide composition, wherein the alkali metal silicide compositionreacts exothermically with the water causing a controlled burn, therebyremoving the volatile or flammable substance.

In addition, the invention relates to a method of removing a volatile orflammable substance in a controlled manner, the method comprising thesteps of exposing the volatile or flammable substance to an alkali metalsilicide composition, and exposing the alkali metal silicide compositionto water, wherein the alkali metal silicide composition reactsexothermically with the water causing a controlled burn, therebyremoving the volatile or flammable substance.

Furthermore, the invention relates to a method of making an alkali metalsilicide composition comprising the steps of mixing an alkali metal withpowdered silicon in an inert atmosphere and heating the resultingmixture up to a temperature below about 475° C., wherein the alkalimetal silicide composition does not react with dry O₂.

In these embodiments, the alkali metal silicide composition may besodium silicide, such as Na₄Si₄, potassium silicide, such as K₄Si₄, andthe like. Also, the exothermic reaction between the alkali metalsilicide composition and the water may consume or clean the volatile orflammable substance.

Moreover, the invention relates to a method for producing hydrogen gascomprising the step of contacting any of the alkali metal silicidecompositions described herein with water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a Differential Scanning Calorimetry (DSC) patternillustrating exothermic reactions between a mixture of Na and Si.

FIG. 2 shows a powder X-ray diffraction (XRD) pattern of a sodiumsilicide composition of the invention and Na₄Si₄ prepared byconventional methods.

FIG. 3 shows a powder X-ray diffraction (XRD) pattern of a sodiumsilicide composition of the invention.

FIG. 4 shows a powder X-ray diffraction (XRD) pattern of NaSi preparedby conventional methods.

FIG. 5 shows a solid state ²³Na MAS NMR spectra for a sodium silicidecomposition of the invention and a sodium silicide composition preparedby conventional methods.

FIG. 6 shows a Differential Scanning Calorimetry (DSC) patternillustrating exothermic reactions of the NaSi product of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As is shown in the attached FIGS. 1-6 and described herein, theinvention relates to alkali metal silicide compositions comprising theproduct of mixing an alkali metal with silicon in an inert atmosphereand heating the resulting mixture to a temperature below about 475° C.,wherein the alkali metal silicide composition does not react with dryO₂. According to the processes described herein, the resultingcomposition can be used as a source of hydrogen by contacting thecomposition with water. While any alkali metal, including sodium (Na),potassium (K), cesium (Cs), or rubidium (Rb) may be used, it ispreferred that the alkali metal used in the alkali metal silicidecomposition be either sodium or potassium. In addition, any type ofsilicon, powdered silicon, or crystalline powdered silicon may be used,for example, powdered crystalline silicon (Alfa Aesar, 325 mesh). Thetheoretical H₂ yield from a sodium silicide composition of the inventionis approximately 0.098 kg H₂/kg NaSi, more than double the Department ofEnergy (DOE) 2005 target for hydrogen fuel sources and larger than the2015 target of 0.081 kg H₂/kg fuel. Accordingly, sodium is the mostpreferred alkali metal, and sodium silicide is the most preferred alkalimetal silicide composition.

FIG. 1 illustrates a pair of exothermic reactions that occur between Naand Si in two temperature regions using a Differential ScanningCalorimetry (DSC) display. The DSC results were obtained using aShimadzu DSC-50 instrument, and the experiment was conducted in a sealedcopper vessel. During the experiment, a mixture of stoichiometricamounts of sodium metal and silicon were combined and heated to about550° C. In general, it is preferred that sodium metal and the silicon bemixed at a 1:1 stoichiometric ratio, or in a mixture having a slightexcess of silicon. At the conclusion of the experiment, there was aslight Na coating on the glass of the flask in which the reactionsoccurred. In addition, it is believed that about 80% of the Na reactedwith the silicon during the experiment.

First run 101 shows the presence of a first exotherm extending fromapproximately 300-450° C. resulting in the release of about 1.0kilojoule of heat per gram (kJ/g) of Na used. The exotherm comes to apeak at about 420° C. and returns to the baseline at about 472° C., atwhich point an endotherm begins to occur. The endotherm, which extendsfrom about 472-505° C. has a trough at about 500° C., is believed to beindicative of a leeching of alkali metal from the material. A secondexotherm extends from about 505-560° C., and results in the dissociationof NaSi to yield sodium metal in the amount of about 0.25 kJ/g of Naused. The dissociation of Na is believed to be the product beingpyrophoric in nature because of the presence of Na metal on the surfaceof the NaSi material.

Thus, the DSC results of FIG. 1 clearly show that one or two reactionsoccur to form one or more of the silicides of sodium. After first run101 was completed, the resulting material was re-heated a second timeunder the same conditions, with the results shown as second run 102.Second run 102 does not show a melting endotherm of Na at 98° C. asexpected if sodium were released at approximately 550° C. in the firstreaction, but does show a complex series of further reactions. The lackof a significant melting endotherm is attributed to the reaction beingslow in the DSC cup, so that the decomposition reaction does not havetime to occur. Evidently though, the initial formation of NaSi isfollowed by other reactions of various complexities.

FIGS. 2 and 3 show powder X-ray diffraction (XRD) patterns of an alkalimetal silicide composition material of the invention. The powder X-raydiffraction patterns were obtained using a Rigaku 200B X-raydiffractometer using a copper source. In particular, FIG. 2 shows thepowder X-ray diffraction pattern of Sample Ni—Si-4 (annealed at about400° C.) in a 0.7 mm diameter capillary tube. FIG. 3 shows a powderX-ray diffraction pattern for an additional sample.

All of the peaks of Na₄Si₄ as calculated from the known crystalstructure are present in the experimental pattern. (See Witte, J.;Schnering, H. G., “The Crystal Structure of NaSi and NaGe (in German)”Zeit Anorgan Allege Chemie 1964, 327, 260-273.). The top pattern in FIG.2, experimental pattern 201, and the pattern in FIG. 3 indicate thepresence of Na₄Si₄ or a closely related allotrope, in addition to theformation of a different material, a mixture of Na₄Si₄, unreactedsilicon, and other unknown products that yield at least seven extrapowder diffraction lines when compared to the bottom pattern in FIG. 2,literature pattern 202, and the pattern of FIG. 4, which are powder XRDpatterns for conventional NaSi. Four of the new peaks at 2Theta anglesof about 18.2, 28.5, 29.5 and 33.7 are shown in experimental pattern 201FIG. 2, and six of the new peaks at 2Theta angles of about 28.5, 29.5,33.7, 41.2, 47.4, and 56.2 are shown in FIG. 3. The presence of theseadditional peaks indicates that the sodium silicide of the invention isdifferent from known sodium silicides found in the literature.

FIG. 5 shows a comparison of the solid state ²³Na MAS NMR spectrum ofNaSi prepared by the methods of the invention and sodium silicideprepared by the methods given in the literature. (See He, J.; Klug, D.D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S., “NMR andX-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem. B 2001,105.). The solid state ²³Na MAS NMR spectrum was obtained using a VarianVXR-400S Spectrometer. As is evident in the figure, the shape andchemical shift behavior are very different from those reported by theliterature. Id. In particular, the NMR spectra shown in FIG. 5 show thata peak 501 for the composition of the invention occurs at approximately18 ppm, while the peak 502 for the sodium silicide material taught bythe literature has a peak at approximately 52 ppm. Clearly, theenvironment of most of the Na⁺ is different in the two preparations. Thenormal chemical shift and symmetry of the spectrum in the composition ofthe invention suggests a more symmetric environment for Na⁺ than in theNa₄Si₄ samples prepared elsewhere by a different method. This differencein chemical shift definitively established the existence of a differentcomposition than that taught by the literature. Furthermore, it shouldbe noted that an earlier solid state ²³Na MAS NMR study observed a verybroad peak near the chemical shift reported in He and very differentfrom that observed with the composition of the invention. (See Gryko,J.; McMillan, P. F.; Sankey, O. F. “NMR Studies of Na Atoms in SiliconClathrate Compounds” Phys. Rev. B, 1996, 54, 3037-3039, and He, J.;Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse, J. S.,“NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J. Phys. Chem.B 2001, 105.). The single rather narrow NMR peak observed duringexperimentation of the composition of the invention at the normalposition of Na+ suggests that in the samples of the composition of theinvention, the sodium ions are in a rather symmetrical environment.

FIG. 6 shows two DSC traces starting with the material of the inventioncomprising the product of the reaction of Na with an equimolar amount ofpowdered Si, heated to 400° C. overnight. The first run 601 shows nomelting endotherm of sodium and no significant exotherms up to 500° C.,above which there is a significant exothermic reaction, probably therelease of Na from Na₄Si₄ at about 550° C. This is confirmed by thesecond run 602, which shows an endotherm due to the melting of Na (abouthalf of that initially used) and a substantial exotherm starting at 300°C. as a result of reaction between the released Na and the productformed by its release in the previous run.

To prevent the formation of the other materials at higher temperatures,the composition of the invention is created by heating an alkali metaland silicon mixture to a temperature below about 475° C., and mostpreferably, to a temperature of about 400° C., which appears to be theoptimal temperature for the formation of the Na₄Si₄ composition of theinvention. Compositions made at higher temperatures gave lower yields ofhydrogen when reacted with water. In addition, the stability of thecomposition of the invention in dry air suggests that the proceduresdescribe herein result in a “protected” product. This protection islikely due to the formation of a SiO₂-like coating on the material.

Accordingly, the invention relates to a method of making an alkali metalsilicide composition comprising the steps of mixing an alkali metal withpowdered silicon in an inert atmosphere and heating the resultingmixture up to a temperature below about 475° C., wherein the alkalimetal silicide composition does not react with dry O₂. In thisembodiment, the step of heating may be staged heating occurring over anextended period of time period of hours comprising the steps of heatingthe resulting mixture up to about 150° C., heating the resulting mixturefrom about 150° C. up to about 200° C., heating the resulting mixturefrom about 200° C. up to about 250° C., heating the resulting mixturefrom about 250° C. up to about 300° C., and heating the resultingmixture from about 300° C. up to a temperature below about 475° C. Thestep of heating the resulting mixture from about 300° C. up to atemperature below about 475° C. more preferably comprises heating theresulting mixture from about 300° C. up to a temperature of about390-425° C., and most preferably comprises heating the resulting mixturefrom about 300° C. to a temperature of about 400° C.

The invention also relates to a method of removing a volatile orflammable substance in a controlled manner, the volatile or flammablesubstance being in the presence of water, the method comprising exposingthe volatile or flammable substance to an alkali metal silicidecomposition. In this embodiment, the alkali metal silicide compositionreacts exothermically with the water causing a controlled burn, therebyremoving the volatile or flammable substance. In addition, the inventionrelates to a method of volatizing, driving off, consuming, converting toa water-miscible species, or otherwise cleaning a volatile or flammablesubstance in a controlled manner, the method comprising the steps ofexposing the volatile or flammable substance to an alkali metal silicidecomposition, and exposing the alkali metal silicide composition towater. In one embodiment, the alkali metal silicide composition reactsexothermically with the water causing a controlled burn, therebycleaning the volatile or flammable substance. Moreover, the inventionrelates to a method of consuming a volatile or flammable substance in acontrolled manner, the method comprising the steps of exposing thevolatile or flammable substance to an alkali metal silicide composition,and exposing the alkali metal silicide composition to water. In anotherembodiment, the alkali metal silicide composition reacts exothermicallywith the water causing a controlled burn, thereby consuming the volatileor flammable substance. In each of the above exemplary embodiments, itis preferred that the alkali metal silicide composition be sodiumsilicide. In addition, the applicability of the above-described methodsis most apparent with respect to the cleaning of volatile and flammablesubstance which cannot be easily cleaned by conventional means, butinstead require alternative means for cleaning, for example, chemicalcleaning.

The material of the invention may be used to clean any non-misciblevolatile or flammable materials, including oils, fuels, etc. Forexample, the material of the invention may be applied to an oil spill ina body of water. When the material of the invention, such as sodiumsilicide, contacts the surface of the water upon which the spill islocated, the material reacts exothermically with the water causing acontrolled burn. The ignition may cause the spill to ignite, thuscombusting the spilled oil and cleaning the spill. This use isparticularly advantageous because the amount of the material of theinvention used is not critical. After an initial amount of the materialis applied to start the combustion, additional amounts may be applied asneeded to further combust the spilled substance until the desired amountis removed. The material may be applied to such as spill in many ways,for example, by spraying or dropping out of an airplane in a mannersimilar to crop-dusting, or from a helicopter.

In addition, the material of the invention may be placed within a watersoluble storage container, for example, a water-soluble pouch, or beimbedded or encapsulated in any sort of containment material, forexample, foams, polymers, porous materials, and the like, which canprovide a time-released reaction with the water by delaying exposure ofthe material to the water. In this manner, the material may be deliveredto a spill in a body of water, for example, an oil spill, by a boat,which could then leave the area prior to the start of combustion.

In addition, the material of the invention may be used to clean volatileor flammable substances in a dry environment. In this case, the materialof the invention may be added to the volatile or flammable substance,for example, applied to the surface of the substance. Then, water may beintroduced, for example, by spraying, to start the reaction of thematerial with the water and initiate combustion of the substance.Similarly, the water may be added to the volatile or flammable substancefirst, and then the addition of the material of the invention willinitiate the combustion.

The alkali metal silicide compositions of the invention vary fromfree-flowing, amorphous gray-black powders, in which the particles havea dull surface, to hard particles having a diameter of about 1-5 mm. Theend product varies depending on the method of preparation. The ease ofhandling of the product, its low moisture absorption from the air, andits rapid reaction with water to produce hydrogen combine to make thismaterial a convenient source of high yields of pure hydrogen.

EXAMPLES

In each of the following examples, evacuable Erlenmeyer flasks were usedto prepare the samples of the alkali metal silicide material. Thesilicon was inserted into the flask, which was then attached to a vacuumline with an UltraTorr fitting and a Kontes vacuum valve. The flask wasthen heated with a flame under vacuum and inserted into a helium-filledglove box, where sodium pieces were added. After removal from the box,the flask was again evacuated and the sodium pieces were melted. Whenthe pressure stabilized at about 3×10⁻⁵ torr, the tubing was sealed offabove the flask and the flask and its contents were heated in a furnaceovernight or longer at the desired reaction temperature. Upon completionof the reaction, the flask was again inserted into the glove box, thetubulation was broken open and the contents removed for storage andfurther study.

Example 1 Initial Experimentation

Although Na is known to react with borosilicate glass at temperaturesabove 300° C., thereby causing darkening, powdered or solid productswere prepared by heating equimolar mixtures of Na and crystallinepowdered Si in evacuated 50 and 125 mL Erlenmeyer flasks. Overnightheating at 500° C. yielded gray-black powders with some crushable lumps.Three separate preparations made at this temperature yielded productsthat were found to release H₂ in amounts equivalent to 0.042, 0.054, and0.058 kg H₂ per kg of sample. Accordingly, it was determined that theconditions of temperature and composition may not be optimal. Theliterature and the DSC results in FIGS. 1 and 6 suggest that 500° C.might be too high a temperature. Accordingly, a sample was prepared byheating the mixture of sodium and silicon up to about 400° C. Theresulting product gave a hydrogen yield equivalent to 0.072 kg H₂ per kgof sample. This yield exceeds the target yields of hydrogen proposed bythe DOE for both 2005 and 2010 and is nearly equal to the year 2015target. These results are very advantageous, especially in light of thesimplicity of the preparation and the air stability of the product.

The material obtained by heating to about 400° C. produces both asolution and a black insoluble material, suggesting that the reaction ofNa with Si is not complete under the stated conditions, as pure NaSiwould be expected to produce products that are completely soluble inwater according to a reaction such as:2NaSi(s)+5H₂O(l)→Na₂Si₂O₅(aqueous)+5H₂↑

Accordingly, it was determined that it might be possible to recover theunreacted Si and react it with more Na, thus increasing the ultimateyield to nearly 100% based on the Si used. To test this possibility, theresidue was recovered, approximately 0.5 g, from the reaction of 1.0grams of the second type of preparation described above with water,dried it, and reacted it with an equimolar amount of sodium at about400° C. The resulting material yielded a quantity of hydrogen equivalentto 0.023 kg H₂ per kg of sample. Thus, the total yield from the originalpreparation was about 0.10 kg H₂ per kg of sample. Recovery of theunreacted material from the initial preparation of Na⁺ Si is clearlypossible.

The product of the reaction between Na and Si in borosilicate glass at400° C. is free of sodium. This is further evidenced by the lack of apeak due to the presence of metallic sodium shown in the solid state²³Na MAS NMR spectrum of FIG. 5. The DSC of the product of reaction ofNa with Si at this temperature shown in FIG. 6 shows no meltingendotherm of Na. Instead, it only shows an exothermic peak at about 500°C. The repeat run clearly shows that this high temperature reactionproduces sodium metal, in agreement with literature results. (Cros, C.;Pouchard, M.; Hagenmueller, P., “Two new Phases of the Silicon-SodiumSystem. (in French)” C. R. Acad. Sc. Paris 1965, 260, 4764-4767., andHe, J.; Klug, D. D.; Uehara, K.; Preston, K. F.; Ratcliffe, C. I.; Tse,J. S., “NMR and X-ray Spectroscopy of Sodium-Silicon Clathrates” J.Phys. Chem. B 2001, 105.).

Example 2 Stability in Air

The Na—Si material of the invention reacts immediately with water toproduce hydrogen and release heat in the process. However, the materialis completely unreactive toward dry oxygen over a period of at least oneday. Unless the relative humidity is high, the powder can be weighed inair or transferred from one container to another. A sample was exposedto laboratory air in an aluminum weighing dish and only slowly reactedwith moisture. After two hours, a small amount of liquid water wasadded, and the black pieces immediately evolved hydrogen. It is likelythat the methods described herein for preparation of the composition ofthe invention result in an alkali metal silicide that is protected by asurface layer of silicon dioxide or some other composition. In anyevent, the resulting material is easy to handle in air, which results inthe ability to produce hydrogen on demand.

These results demonstrate that it is straightforward to produce a stablepowdered or granular material that likely contains a silicide with thestoichiometry NaSi, together with an unknown amount of other substances(possibly glassy SiO₂ and unreacted silicon). The product, while stablein dry air and only slowly reactive in moist air, produces large yieldsof hydrogen when introduced into liquid water. The gaseous product ispure hydrogen, uncontaminated with anything except water vapor and smallamounts of silanes such as SiH₄. Thus, the material is an excellentsource of hydrogen for use in fuel cells.

Example 3 Reaction Between Sodium and Silicon Powder at 500° C.

Sample Na—Si-1 was prepared by introduction of 0.56 g of powderedcrystalline silicon (Alfa Aesar, 325 mesh) into an evacuable ErlenmeyerFlask. After outgassing under vacuum with a gas-oxygen torch (˜300° C.).the flask was put into a helium-filled glove box and 0.46 g. of sodiummetal was added. After evacuation and melting of the sodium, the stem ofthe flask was sealed off under vacuum and the flask and contents wereheated in a furnace for 2 hrs at 300° C., 22 hrs at 400° C. and 48 hrsat 500° C. The Pyrex flask became dark brown-red in the process and theproduct consisted of both powder and small hard lumps. In the glove box,0.66 g of product was recovered. A 24.5 mg sample produced 0.517millimoles (mmol) of hydrogen upon addition of water. This correspondsto 0.0423 kg of H₂ per kg of sample. If the hydrogen is produced onlyfrom sodium silicide, the amount corresponds to 43% Na₄Si₄.

Example 4 Recovery of Residue from the Product of the Invention

The second preparation of Na⁺ Si (Sample Na—Si-2) corresponded to 55%Na₄Si₄ according to the yield of hydrogen. A 1.0 g sample of the productwas reacted with water in a nitrogen-filled glove bag, with copiousamounts of hydrogen produced. The reaction left a black residue thatcould not be recovered by centrifugation because slow evolution of gascontinued to cause mixing. The continued evolution of hydrogen would beexpected in this basic solution if the residue contained elementalsilicon. The product was neutralized with HCl, washed by repeatedcentrifugation and dried. The resulting black powder (0.49 g) was againreacted with Na at 500° C. and produced 0.021 kg of H₂ per kg of sample.

Example 5 Preparation of the Highest Yielding Sample (Sample Na—Si-4)

As shown in FIG. 1, it became apparent from the Differential ScanningCalorimetry (DSC) experiments on the heat evolved in the reaction of Nawith Si that there were two exothermic processes occurring. It isbelieved that, after formation of Na₄Si₄ at around 400° C., furtherheating caused dissociation of the product with the formation of Nametal and other silicides. This result was somewhat surprising sincepure Na₄Si₄ is generally prepared at or above 500° C. To test whetherpreparation at 400° C. instead of 500° C. increases the yield of theproduct of the invention, a sample was prepared as described in Example3, except that the flask and its contents were heated to 400° C.overnight. The resulting product had fewer lumps and gave a hydrogenyield that corresponded to 73% Na₄Si₄. The DSC of this sample shown inFIG. 6 confirmed the formation of sodium upon heating to 560° C. andshowed that an exothermic reaction resulted upon re-heating, which isbelieved to be the result of re-formation of sodium silicide.

In particular, FIG. 6 shows Differential Scanning Calorimetry (DSC)traces of the reaction of 4.9 mg of Na with 5.0 mg of Si. The mass of Nawas determined from the measured heat of melting. It is likely that theexotherm with a peak at 400° C. is due to the formation of Na₄Si₄ andthat further heating causes a secondary reaction. As a result, secondrun 602 shows further reactions of the various products formed in firstrun 601. Second run 602 shows no melting endotherm of free sodium. Thiscontrasts with the behavior of a pre-formed sample, annealed at 400° C.which releases Na when heated to 550° C. in the DSC cup. It is likelythat the reaction to form Na₄Si₄ in the DSC cup is slow, so that thesecond reaction to release sodium does not have a chance to occur duringthe short time of the DSC experiment.

Example 6 Preparation of Potassium Silicide (KSi) Material

The KSi material was produced by first mixing stoichiometric amounts ofpowdered (350 mesh) Si from Sigma-Aldrich with K metal chunks in anErlenmeyer flask, equipped with a stem to attach to the vacuum line.This was done in a He-filled glove box. The He was pumped off and themixture heated with a flame until the K melted and the system wasoutgassed to ˜10-5 Torr, at which point the Pyrex stem was sealed-offwith a flame. The Erlenmeyer was then put into a muffle furnace andheated for about two hours each at 150° C., 200° C., 250° C., 300° C.,and 350° C. and then overnight at 400° C. The tube was broken open inthe glove box and the product was scraped off the walls. The materialconsisted of powder and chunks and the latter were ground up into a finepowder. A sample removed from the glove box was poured into an aluminumweighing dish and exposed to laboratory air. It reacted slowly, if atall, with no heating or flame. But when a bit of powder was dropped intoa beaker of water it ignited immediately.

The yield of hydrogen upon reaction with water indicated only about a 50percent (50%) conversion to KSi, which indicates that a longer reactiontime or better agitation during preparation is necessary. Accordingly,it is expected that optimization of the conditions of synthesis willlead to yields comparable to those achieved with the sodium silicidecompositions of the invention.

1. An air stable sodium silicide composition having a powder X-raydiffraction pattern comprising peaks with 2Theta angles of about 18.2and about 41.2 and at least one peak with a 2Theta angle selected fromabout 28.5, 29.5, 33.7, 47.4 and 56.2.
 2. The air stable sodium silicidecomposition of claim 1, wherein the molar ratio between the sodium metaland the silicon is about 1:1.
 3. The air stable sodium silicidecomposition of claim 1, wherein the sodium silicide composition does notreact with dry O₂.
 4. A method for producing hydrogen gas comprising thestep of contacting a sodium silicide composition of claim 1 with water.5. An air stable sodium silicide composition having a single peak in thesolid state ²³Na MAS NMR spectra at about 18 ppm.
 6. The air stablesodium silicide composition of claim 5, wherein the molar ratio betweenthe sodium metal and the silicon is about 1:1.
 7. The air stable sodiumsilicide composition of claim 5, wherein the sodium silicide compositiondoes not react with dry O₂.
 8. A method for producing hydrogen gascomprising the step of contacting a sodium silicide composition of claim5 with water.